Here is a bit more detail on one of the samples they studied:

The researchers found the shock lamellae in glassy fulgurites in Les Pradals, in southern France. The fulgurites looked like wet, dark algae and formed in the crevices of a jagged fracture that sliced through a granite rock outcrop there.

And here is a somewhat related video that is more cool than anything else.   Someone recorded this and uploaded sometime last year:

Using a GoPro camera suction cupped to the hood of my car, I captured this video of a lightning storm near Motley, Minnesota on Sunday, July 6th, 2014 starting at 5:45 a.m.

We hope you like the video.  And if nothing else, don’t stand next to any large rocks, poles and get out of the water if you see a lightning storm forming.

thanks to livescience.com for the great info